180 days
You said: "As per AC21, the INS is agnostic to who your employer is after you have worked for the sponsoring entity for 180 days..."
I\'d love to hear where you believe there is a requirement to work for the petitioning employer for even a single day in order to take advantage of AC21 portability of immigration visas after 180 days.
Taking AC21 out of the mix, there was never any requirement to work for the petitioning employer for a single day BEFORE the GC was approved. Afterwards, however, is a different story. All that AC21 did, was to eliminate the requirement that the beneficiary work for the petitioner after the I-485 was approved, if it had been pending for more than 180 days.
The I-485 is the only step of the Green Card that YOU initiate, which means that when you file it, YOU must have the intent to work for the employer. If the employer no longer wishes to hire you, under the law they have a simple legal step to indicate this - they write the INS a letter requesting withdrawal of the approved I-140. If they do not do this, in the eyes of the law they are still intending to hire you in the future, and you can safely file the I-485 so long as YOU intend to work for them. After 180 days, you can change your mind.
I am NOT an attorney, but this is based on my reading of the law and the implementation regulations of AC21.